TOF sensors (Time of Flight sensors) emit light and use the reflected light to determine the distance covered by the light from a reflection surface back to the sensor. This can be achieved, for example, by measuring the time required by a light pulse to pass from a source, via a reflection surface, back to the sensor. Another possibility is to modulate an oscillation onto the light and to compare the phase of the oscillation of light which has been reflected back with the phase of light which has been emitted. The length of the light path can be determined from the phase difference with knowledge of the modulation frequency.
Such sensors can be used to monitor, for example, door systems, for example of an elevator, in order to prevent items or people getting caught in the door.